Field of the Invention
Embodiments of the invention generally relate to water desalination. More particularly, embodiments of the invention relate to systems and methods for the production of sweet water (hereinafter also referred to as “treated water”), from produced brine available at crude oil and natural gas production facilities.
Description of the Related Art
Sweet water is used in a Gas Oil Separation Plant (hereinafter referred to as “GOSP”) to remove salt from crude oil. Traditionally, water is provided from an external source, for example, a distant aquifer or a seawater desalination plant, and treated using a desalination system, for example, a thermal desalination system, such as a multiple-stage flash distillation system, a multiple-effect distillation system, or a vapor compression system, as non-limiting examples.
Currently, various commercial processes are employed in steam generation for the sole application of steam assisted gravity drainage (SAGD) of an oil reservoir. This application was first mentioned as an example in 1989. The process was further developed in the early 2000's to use vapor-compression evaporators and has been continuously improved.
FIG. 1 is a schematic diagram of a conventional thermal desalination system: a multiple-stage flash distillation system. The multiple-stage flash distillation system, as shown in FIG. 1, includes multiple evaporating chambers, the coldest of which receives sea water in a feed stream. The sea water flows through a series of heat exchanger tubes and is heated at each stage before reaching an external heat exchanger, for example, a brine heater—labeled as “Heating steam” in FIG. 1. The brine heater overheats the feed water, as compared to the temperature and pressure in the first stage, to vaporize the feed water when the feed water enters the first evaporating chamber. Successive evaporating chambers in this conventional system are operated at successively decreasing levels of pressure from the first stage at the hot side (i.e., left-hand side of FIG. 1) to the last stage at the cold side (i.e., right-hand side of FIG. 1). The produced steam in each stage is condensed into fresh liquid water in a heat exchanger arranged on the top of a respective evaporating chamber. The brine is circulated to the next stage for further evaporation until it reaches the coldest stage, and then subsequently exits the system. The fresh water produced from each stage is commingled in a “distillate” line and exits the system through the coldest stage.
FIG. 2 is a schematic diagram of another conventional thermal desalination system: a multiple-effect distillation system. The multiple-effect distillation system, as shown in FIG. 2, operates in a mode similar to the multiple-stage flash distillation system shown in FIG. 1, in that successive cells are operated at decreasing levels of pressure and temperature from the first hot chamber (i.e., left hand-side of FIG. 2) to the last cold chamber (i.e., right hand-side of FIG. 2). According to this conventional system, each chamber contains a horizontal hot tube bundle that is sprayed with seawater. Heating steam flows inside the tubes of the hot tube bundle, whereby steam produced from one stage is conveyed to the next stage at a lower temperature and pressure to be condensed into liquid treated water. The steam produced in a given stage from seawater evaporation is at a higher temperature than the next stage, and therefore can be used as a heating fluid for the following stage. At the outlet of the final condenser stage, part of the warmed seawater flow can be used for recirculation at the inlet of the process, while the other part is usually discharged at sea. Brine and distillate are collected at the last stage in the process and pumped out of the multiple-effect distillation system.
FIG. 3 is a schematic diagram of another conventional thermal desalination system: a vapor compression system. The vapor compression system, as shown in FIG. 3, is generally used when no heat is available from an external source. The principle of operation of the vapor compression system is very similar to the multiple-effect distillation system shown in FIG. 2. However, the primary advantage of a vapor compression system is that steam is extracted from the last stage and compressed to a pressure above the first (i.e., hottest) chamber pressure. This allows the steam to be re-used as a heat source in the first stage chamber of the vapor compression system.
Each of these conventional thermal desalination systems requires a complex infrastructure to deliver fresh water from an external source and consumes large amounts of external energy to produce treated water for use in a GOSP facility. For example, conventional desalination plants are characterized by an energy consumption of 4 kWh/m3 of desalinated sea water to which the energy cost of transporting the desalinated water to a site of use must be added. Therefore, what is needed are improved systems and methods for producing treated water from produced brine available at crude oil and natural gas production facilities, which operate at reduced costs, for example, construction, operation, and maintenance costs, and require less external energy to generate the treated water (i.e., due to the elimination of pipeline infrastructure and systems for supplying fresh water to the GOSP facility from a remote location).